Parameterisations of fractional cloudiness in large-scale atmospheric models rely on information about the subgrid-scale variablity of the total water specific humidity, qt , provided in form of a probability density function (PDF). In this contribution, four different approaches to evaluate such total-water PDFs are discussed: (i) Satellite spectroradiometers with high spatial resolution allow to construct at the scale of model grid boxes a histogram, and subsequently to derive the moments of the PDF, of the vertical integral of qt . This can be compared to the same quantity diagnosed from the model parameterisation. Although the vertical integral mostly focuses on the boundary layer, and involves issues in grid-boxes with orographic variability, it allowed nevertheless in the example
presented to pinpoint deficiencies of a model parameterisation. (ii) Assuming a simple PDF shape and saturation within clouds, the simple “critical relative humidity” metric can be derived from infrared sounders and/or cloud lidar in combination with reanalysis data with a vertical resolution. It allows to evaluate the underlying PDF of any cloud scheme, but is sensitive to the assumptions. (iii) Supersites with a combination of ground-based lidar, radar and microwave data provide high-resolution high-quality reference data. In a “virtual reality” framework, we showed, however, that it is difficult to evaluate higher moments of a spatial PDF with this temporally-varying data. (iv) From a hierarchy of models from general circulation models to direct numerical simulations, we find
that the variance of the qt follows a power-law scaling with an exponent of about -2. This information is very useful to improve the parameterisations.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:13993 |
| Date | 26 November 2015 |
| Creators | Quaas, Johannes, Grützun, Verena, Schemann, Vera, Weber, Torsten |
| Contributors | Max-Planck-Institut für Meteorologie, Helmholtz-Zentrum Geesthacht, Universität Leipzig |
| Publisher | European Centre for Medium-Range Weather Forecasts |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:article, info:eu-repo/semantics/article, doc-type:Text |
| Source | ECMWF Workshop proceedings (2013) S. 89-91 |
| Rights | info:eu-repo/semantics/openAccess |
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